Inorganic-organic frameworks, particularly metal-organic frameworks (MOFs), have attracted considerable attention over the last decade because they exhibit a wide range of interesting physical properties.1 These include the ability to store or separate gases and perform catalytic reactions in nanoporous MOFs; while fascinating magnetic and electronic properties are found in denser frameworks. The transition metal gallate frameworks, M(C7HxO5)·2H2O (M = metal and x = 3 or 4) have significant pore space and long-range magnetic order.2,3 Interestingly the four gallate compounds reported adopt the same structure, although the Fe framework contains Fe3+ while the other materials have Mn2+, Co2+ or Ni2+. This similarity is odd, since to retain overall charge neutrality, the framework must lose an additional proton (hydrogen ion) during synthesis to accommodate Fe3+ and it is expected that this would have a significant effect on the structure.
Figure 1:The structure of Ni gallate showing a) the gallate ligand and b) the NiO6 chains and two distinct water molecules. In a) the oxygen atoms in the carboxylate, meta and para groups are red and labelled Oc, Om and Op, respectively. The Ni, C and H atoms are coloured green, black and gray while the oxygens of the two water molecules are blue and orange.
To understand how these fascinating frameworks accommodate cations with different charges we have recently investigated their structures using a combination of synchrotron X-ray and neutron powder diffraction. This approach takes advantage of the very high resolution and intensity offered by synchrotron X-ray diffraction while using the higher sensitivity of neutron diffraction to light elements to obtain the hydrogen positions more accurately. X-ray diffraction patterns were obtained using beamline I11 at Diamond Light Source while neutron diffraction was done at the OPAL reactor in Sydney, Australia. Structures were refined using the Rietveld method within the Fullprof program.4
Analysis of the structures of the divalent gallates showed that charge balance is achieved in these frameworks by deprotonation of the carboxylate and para-hydroxy groups of the gallate ligand, while both meta-hydroxy groups are protonated (see Fig. 1a). Very weak additional reflections were observed in the low temperature diffraction patterns of the divalent compounds, which was possible due to the very high quality data obtained (see Fig. 2). This indicated that the divalent gallates undergo a phase transition to a structure with the same symmetry, but with the c-axis doubled. The transition seems to be caused by the rearrangement of the pore water leading to the presence of two distinct water molecules in the low temperature structure, as opposed to one in the higher temperature phase (see Fig 1b). This strengthens the hydrogen bonding between one of the water molecules and the hydrogen of one of the meta-hydroxy groups at the expense of slightly weakening the hydrogen bonding with the other water molecule (c.f. bond distances of 1.58(4) Å and 2.26(4) Å in the low temperature structure of Co gallate to 1.99(3) Å in the high temperature phase). The high resolution of beamline I11 enabled highly precise lattice parameters to be determined at different temperatures. This established that the phase transition is discontinuous and, in Co gallate, occurs above 225 K.
Figure 2:100 K synchrotron X-ray diffraction pattern of Co gallate. The crosses, upper and lower continuous line are the experimental, calculated and difference profiles. The insert shows the most intense reflections (arrowed) indicative of the doubling of the c-axis.
K-edge XANES spectroscopy, obtained using beamline B18 at Diamond Light Source, confirmed that only Fe3+ is present in Fe gallate and all other cations are purely divalent (see Fig 3.). Doping Fe gallate with any of the divalent cations does not change the charges of the cations present, with Fe remaining trivalent at all compositions and all other cations being divalent. The presence of cations with different valencies disordered on the same site of a framework material is highly unusual and has implications for their electronic conductivity.
The structure refined for Fe3+ gallate shows that half of the meta-hydroxy groups are deprotonated to accommodate the higher cation charge. The meta-hydroxy oxygen atoms have a much shorter bond, 2.031(3) Å, with the Fe3+ cations compared to an average of 2.233(5) Å and 2.136(3) Å in Co2+ and Ni2+ gallates. This stronger bond stabilises the higher oxidation state of Fe3+ and weakens the bond between this oxygen and its hydrogen. This seems to lead to the hydrogen being closer to, and bonding more strongly with, the lattice water than in the other gallates. This behaviour is analogous to acid hydrolysis of simple Fe(III) salts.5 The water molecules in Fe gallate are disordered and refinements indicate that, unlike the divalent gallates, this compound does not undergo a phase transition at low temperatures. It is likely that increased interaction between the water molecules and the meta-hydroxy hydrogen is the cause of the disorder and that this prevents the low temperature phase transition.
Figure 3: K-edge XANES spectra of Fe and Ni (insert) gallate and, for comparison, divalent and trivalent standards.
In summary we have found that, despite divalent and trivalent gallates initially appearing to be isostructural, the difference in cation charge has a significant effect on their unique architecture, particularly on the positions of the protons and water molecules. The charge of the cations has also been found not to be altered in doped samples leading to structures with a mixture of divalent and trivalent cations, which is highly unusual in inorganic-organic frameworks.
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